Narrow linewidth (low phase noise) lasers are used in a wide range of applications including coherent communication, atomic spectroscopy and a variety of sensors including acoustic sensors. Conventional narrow linewidth lasers employ a range of approaches to achieve line narrowing. Most of these approaches require the alignment of extended cavities to the gain medium of the laser or the formation of other extended cavities in the form of ring cavities. An alternate approach is to employ a fiber laser where the extended cavity is provided by the fiber itself.
In the case of mechanically aligned external cavity lasers, the lasers are both expensive to build and inherently unstable due to mechanical misalignment sensitivity. In the case of the fiber laser, the laser is large in size and expensive due to the configuration of a doped fiber being pumped with a primary laser resulting in significantly reduced efficiency. In both types of lasers the feedback element typically possesses a sharp spectral feature to provide spectral selectivity beyond that of the cavity resonance (e.g., a free-space Littrow grating or fiber Bragg grating.) These known narrow linewidth laser solutions require a relatively high level of optical feedback from the external cavity (typically >10 dB roundtrip attenuation) and a precisely controlled external cavity length.